1. Field of the Invention
This invention relates to a sheet handling apparatus which receives a series of recording sheets discharged from an image recording apparatus such as a copier, a printer or the like on a handling tray in a stack and transfers the sheet stack thus formed to an accumulation tray one by one to stack a plurality of sheet stacks on the accumulation tray.
2. Description of the Related Art
In such a sheet handling apparatus, generally a series of sheets discharged from the image recording apparatus onto the handling tray are lined up to form a sheet stack and the sheet stack is transferred to an accumulation tray, after being stapled as required, so that the operator can easily remove the sheet stack. On the accumulation tray, a plurality of such sheet stacks are stacked one by one.
As disclosed for instance in Japanese Unexamined Patent Publication No. 6(1994)-9142, sheet handling apparatus in which a sheet stack on the handling tray is transferred to the accumulation tray in a direction which intersects the direction in which the sheets are discharged onto the handling tray referred to as "the discharge direction", herein. In the apparatus, the sheet stack is transferred to the accumulation tray by an ejector lever which ejects the sheet stack toward the accumulation tray and a conveyor roller pair which conveys the sheet stack while pinching the sheet stack between a pair of conveyor rollers. Further an apparatus in which a gripper grips a leading end portion of the sheet stack, draws the sheet stack out from the handling tray and transfers it to the accumulation tray is known.
The sheet handling apparatus in which the sheet stack is transferred by rotation of the roller pair is disadvantageous in that it is difficult to transfer and release the sheet stack onto the accumulation tray with the edges of the sheets lined up and that it is difficult, when the sheet stacks are stapled, to stack the sheet stacks so that the staple on the preceding sheet stack does not interfere with the edge of the following sheet stack. Especially as the handling speed increases, the transfer mechanism must operate at a higher speed to keep of the sheet stack lined up and increase the transfer efficiency. Further it is preferred that the transfer mechanism be arranged not to interfere with the operator when the operator takes out the sheet stacks on the accumulation tray.
Further in such sheet handling apparatuses, it is often required to staple the sheet stack. As disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 6(1994)-9142 and 4(1992)-288292, a sheet handling apparatus which is provided with a pair of staplers in order to staple the sheet stack in two positions is known. In the apparatus, at least one of the staplers is arranged to be movable so that the stapling positions on the sheet stack can be changed according to the size of the sheets.
However providing a pair of staplers, at least one of which requires a mechanism for moving, complicates the structure and increases the overall size of the apparatus.
Further an attempt to staple the sheet stack in two positions by a single stapler, by which the sheet stack is first stapled in a first position and then in a second position, the portion may result in of the sheet stack between the first stapling position and the second stapling position bulging.
That is, when the sheet stack is transferred to bring the second position to the stapler after the sheet stack is stapled in the first stapling position, a part of the sheet stack near the staple is pressed by the staple whereas the part of the sheet stack remote from the staple is free. Accordingly when there are formed spaces between the sheets at the part remote from the staple, the sheets near the surface of the sheet stack bulge outward and when the sheet stack is stapled in the second position with the bulge left as it is, a bulge is formed in the bound sheet stack between the first and second stapling positions. Further when the part between the first and second stapling positions of the sheet stack which has been stapled in the first stapling position and is to be stapled in the second stapling position is not straight, the length of the upper surface of the sheet stack between the first and second stapling positions becomes larger than that of the lower surface of the sheet stack and a bulge is formed in the bound sheet stack due to the difference in length. Further when a sheet stack which has been stapled in the first stapling position is conveyed by rollers, the conveying speed fluctuates in the direction of thickness of the sheet stack, which forms a similar bulge due to the fact that the sheet stack has been stapled in the first stapling position, and when the sheet stack is stapled in the second position with the bulge left as it is, a bulge is formed in the bound sheet stack between the first and second stapling positions.
Further when the sheet stack is stapled in a first stapling position remote from the part where it is held and then in a second stapling position nearer to the part where it is held, a bulge is more apt to be formed. However when the sheet stack is transferred to be stapled first in a position nearer to the part where it is held and then in a second stapling position remote therefrom, the sheet stack must be reciprocated back and forth, which increases the stapling time and reduces the throughput of the apparatus.